add Rationale section

Author: skirpichev

peps/pep-0791.rst

@@ -54,6 +54,22 @@ simple wrapper to the ``libm``, as most functions in the
 :external+py3.14:mod:`math` module.
 
 
+Rationale
+=========
+
+Why not fix the :external+py3.14:mod:`math` module documentation instead?
+Sure, we can be much more vague in the module preamble (i.e. roughly say
+that "the :external+py3.14:mod:`math` module contains some mathematical
+functions") and we can accurately describe input/output for each function.
+
+But the major issue is that current module mix different, almost non-interlaced
+application domains.  Adding more documentation will just highlight this and
+make the issue worse for end users (more stuff to read/skip).  And it will not
+fix issue with discoverability (to know in which module to find a function, and
+that it can be found at all, you need to look at all the functions in the
+module), nor with tab-completion.
+
+
 Specification
 =============